1. Field of the Invention
The present invention is a means for increasing the selectivity of a membrane pervaporation aromatics/nonaromatics separation process. Selectivity is unexpectedly increased by recycling part of the permeate to the initial separation process stage for co-processing in said stage with fresh feed. The selectivity obtained is greater than that obtainable by processing either the feed or a recycle permeate stream by itself.
2. Description of the Related Art
It is common in membrane separation processes to recycle either or both of various permeate or retentate streams to the primary separation zone. Similarly, passage of permeate and/or retentate streams to independent downstream permeation zones for additional separate treatment or processing is also fairly common and employed as a means for improving the quality of the desired products.
U.S. Pat. No. 2,985,588 is directed to a method for the start up and shut down of pervaporation process used for separating organic mixtures, including hydrocarbons, in which a feed stream is sent to a first permeation zone producing a first permeate and a first retentate. The first permeate is sent to a second permeation zone wherein it is split into a second permeate and a second retentate. The second retentate is recycled to the first permeation zone while the second permeate is sent to a third permeation zone wherein it is split into a third retentate and a third permeate, said third retentate being recycled to the second permeation stage while said third permeate is recovered as pure product (e.g. benzene). Returning to the first permeation stage, the first retentate from said first permeation stage is sent to separate down stream permeation stage 2A wherein it is split into separate retentate stream 2A and permeate stream 2A, which permeate stream 2A is recycled to the first permeation zone while the retentate stream 2A is sent to get another separate down stream permeation stage 3A wherein it is split into separate retentate stream 3A and permeate stream 3B, which permeate stream 3A is recycled to permeation zone 2A while retentate stream 3A is recovered as product. Permeate from a given permeation stage is never recycled to the input of the same stage.
U.S. Pat. No. 3,370,102 feeds permeate from a first separation stage to a second separation stage wherein said permeate is split into a second permeate stream and a retentate stream. The retentate stream from said second separation stage is recycled to the inlet of the first separation stage. The second permeate stream is fed to a third separation stage wherein it is split into a third permeate stream which is recovered, and a third retentate stream which is recycled to the inlet of the second separation stage.
U.S. Pat. No. 4,617,127 produces low alcohol content beverages by passing a high alcohol content stream to a membrane separation zone to produce an alcohol rich permeate and an alcohol lean retentate. Part of the retentate is recovered while part is recycled to the inlet of the first membrane separation stage for admixture with fresh feed.
U.S. Pat. No. 4,774,365 pervaporatively separates alcohols from ethers. Alcohols constitute the permeate which is subsequently recycled to the inlet of an upstream etherification process for reuse.